Anthropogenic aerosol and cryosphere changes drive Earth’s strong but transient clear-sky hemispheric albedo asymmetry

A striking feature of the Earth system is that the Northern and Southern Hemispheres reflect identical amounts of sunlight. This hemispheric albedo symmetry comprises two asymmetries: The Northern Hemisphere is more reflective in clear skies, whereas the Southern Hemisphere is cloudier. Here we show that the hemispheric reflection contrast from differences in continental coverage is offset by greater reflection from the Antarctic than the Arctic, allowing the net clear-sky asymmetry to be dominated by aerosol. Climate model simulations suggest that historical anthropogenic aerosol emissions drove a large increase in the clear-sky asymmetry that would reverse in future low-emission scenarios. High-emission scenarios also show decreasing asymmetry, instead driven by declines in Northern Hemisphere ice and snow cover. Strong clear-sky hemispheric albedo asymmetry is therefore a transient feature of Earth’s climate. If all-sky symmetry is maintained, compensating cloud changes would have uncertain but important implications for Earth’s energy balance and hydrological cycle.


EDITORIAL POLICIES AND FORMATTING
We ask that you ensure your manuscript complies with our editorial policies. Please ensure that the following formatting requirements are met, and any checklist relevant to your research is completed and uploaded as a Related Manuscript file type with the revised article.
(Please contact the editor if you are unable to make your data available).

All Communications Earth & Environment manuscripts must include a section titled "Data
Availability" at the end of the Methods section or main text (if no Methods). More information on this policy, is available at <a href="http://www.nature.com/authors/policies/data/data-availabilitystatements-data-citations.pdf">http://www.nature.com/authors/policies/data/data-availabilitystatements-data-citations.pdf</a>.
In particular, the Data availability statement should include: -Unique identifiers (such as DOIs and hyperlinks for datasets in public repositories) -Accession codes where appropriate -If applicable, a statement regarding data available with restrictions -If a dataset has a Digital Object Identifier (DOI) as its unique identifier, we strongly encourage including this in the Reference list and citing the dataset in the Data Availability Statement.
DATA SOURCES: All new data associated with the paper should be placed in a persistent repository where they can be freely and enduringly accessed. We recommend submitting the data to discipline-specific, community-recognized repositories, where possible and a list of recommended repositories is provided at <a href="http://www.nature.com/sdata/policies/repositories">http://www.nature.com/sdata/policies/ repositories</a>. If a community resource is unavailable, data can be submitted to generalist repositories such as <a href="https://figshare.com/">figshare</a> or <a href="http://datadryad.org/">Dryad Digital Repository</a>. Please provide a unique identifier for the data (for example a DOI or a permanent URL) in the data availability statement, if possible. If the repository does not provide identifiers, we encourage authors to supply the search terms that will return the data. For data that have been obtained from publically available sources, please provide a URL and the specific data product name in the data availability statement. Data with a DOI should be further cited in the methods reference section.

REVIEWER COMMENTS:
Reviewer #1 (Remarks to the Author): Review of "On the rise and fall of Earth's strong clear-sky hemispheric albedo asymmetry" by Michael Diamond et al.
The motivation for the study is the present-day hemispheric symmetry in TOA reflected shortwave radiation (i.e., planetary albedo), which is a fascinating yet not well understood feature of Earth's climate. If the symmetry was the result of some intrinsic compensation mechanisms, this would put a profound constraint on Earth climate and would have important implications for past and future regional changes. Somewhat in contrast to previous work, the authors focus on the clearsky aspect of the problem, i.e., the quite substantial hemispheric asymmetry in the clear-sky albedo without clouds that makes the all-sky symmetry so fascinating.
The paper is well written and and the analysis methods appear valid. I have already reviewed the previous version of the paper that was under review at Nature. In the revised version considered here the authors present a long rebuttal letter, but the the actual changes to the manuscript in my view are rather small and most of the reviewer's criticism is discussed away (instead of substantial changes to the manuscript). As a result, I remain to feel that the authors are overselling their results and that their work would fit better into a specialized journal, or maybe into a perspective/opinion article. I hope to explain this in the following.
In the previous version I have criticized that the authors have, maybe without wanting to do so, created the idea that previous work has not recognized the leading role of aerosols in setting the clear-sky albedo asymmetry in the present-day climate. In the revised version, the authors now talk of a traditional, most-cited and/or original view that they claim to here find to be insufficient (e.g., L17 and L70 of the tracked-changes manuscript). I find this wording is still trying to create the picture that the authors are here correcting a "wrong" view that in my view simply does not exist. I thus remain unsatisfied with the broader framing of the paper.
The clear-sky albedo diagnostic itself and the finding of its transient nature are interesting, and the combination of satellite observations and climate model analysis (including historic and future warming runs) makes a good case about the (likely) time dependence of the clear-sky asymmetry, although the analysis method is fairly standard and straightforward. The numbers derived here are subject to quite large uncertainties (i.e., the pre-industrial clear-sky asymmetry is estimated to be within ~0-4 Wm-2, which does not seem a strong constraint), but nevertheless these numbers are helfpul reference points.
The paper closes with a discussion about the possible meaning of the transient nature of the clearsky asymmetry. This discussion is necessary speculative as it hinges on the unanswered question of whether the all-sky symmetry is pure chance or an intrincic feature, and so gives little further insights. This part might actually fit better into a perspective or opinion piece instead of an original research article.
Reviewer #2 (Remarks to the Author): The authors investigate clear-sky northern hemisphere and southern hemisphere albedo asymmetry using CERES data and CMIP6 outputs. They separate atmosphere and surface contributions using a simple one-layer model. They show that the atmosphere contributes more than surface to the clear-sky hemispherical albedo asymmetry. As a consequence, when aerosol loading in the northern hemisphere decreases, clear-sky albedo asymmetry is reduced. They show that this is the case using two 15-year periods using CMIP6 runs.
The manuscript is well written and understanding results is straightforward. This is in part, as the authors acknowledged in the introduction section, that a larger contribution of the atmosphere to the clear-sky hemispherical albedo asymmetry has been pointed out in earlier studies. therefore, a smaller clear-sky hemispherical albedo asymmetry as the aerosol loading in the northern hemisphere being reduced is a corollary to the earlier result. Therefore, the results are not entirely novel.
I have three minor comments. The third one can be major.
The authors link this clear-sky hemispherical albedo asymmetry to all-sky albedo symmetry and argue on line 250 that, "without any compensation mechanism, we should observe an asymmetry in all-sky reflection in the next few decades". This is true when clouds do not change. However, because cloud signal is much larger compared to aerosol, all-sky asymmetry can be maintained by a separate mechanism regardless of the size of the clear-sky albedo asymmetry. Earth may have a way to maintain symmetrical hemispherical albedo by manipulating clouds because all-sky energy input to each hemisphere may need to be balanced.
Because the way of authors compute surface and atmosphere contributions, both contributions change when aerosol loading changes. Two contributions are not entirely independent, although the change in the surface contribution is at least one order of magnitude smaller than the change in atmospheric contribution. This can be seen by taking a derivative of Eq. (1) with respect to alpha_atm. The authors need to recognize this point.
If only the transmission changes, such as by decreasing soot loading without changing alpha_atm (i.e. changing absorption coefficient without changing scattering coefficient), then only surface contribution changes. Because of the way authors compute contributions, the change of the contribution by aerosol change depends on what type of aerosol changes. A smaller clear-sky hemispherical albedo symmetry during the 1850-1865 period depends on the type of aerosols used in CMIP6 runs. I doubt that the sensitivity to aerosol type is large enough to changes conclusion. But at lease the authors need to discuss that the contribution is sensitive to aerosol type.
Reviewer #3 (Remarks to the Author): Review of On the rise and fall of Earth's strong clear-sky hemispheric albedo asymmetry, by Diamond et al.
Formulations in the paper have changed from "The traditional explanation…" to "The most cited explanation…" (in the abstract) and "The traditional and most-frequently invoked explanation…" (in the introduction), with references to Vonderhaar and Suomi (1971), Ramanthan (1987), Stephens et al (2015, 2016 and Datseris and Stevens (2021), but this does not solve the problem.
Neither the paper, nor this review of it is a meta-study of the literature and citation paths on this topic, but a brief search shows that this account is at best incomplete.
- Vonderhaar & Suomi (1971) and Ramanathan (1987) indeed point at surface features being compensated by clouds, and are indeed very well cited (more than 200 and more than 400 citations respectively, although likely not only for pointing at the interhemispheric albedo symmetry).
-Stephens et al (2016) (36 citations) also refers to "…the surface radiative fluxes that are slightly more asymmetrical", but without reference to the mentioned Ramanathan (1987) or Vonderhaar&Suomi (1973) in that context. -Stephens et al (2015), as the authors agree, discuss and quantify contributions to clear-sky asymmetry arising from differences in aerosol distribution (their section 4). This paper, that arguably is central for bringing the interhemispheric symmetry into discussion in more recent times, has 191 citations.
-In addition, as the authors also agree but still leave out in introducing their topic and motivating their study, both earlier Voigt et al (2013) (39 citations), and later Bender et al (2017) (11  citations), and Jonsson and Bender (2021) (1 citation) show or clearly state (and stating should be enough to count for a general argument of the type "most cited view") that atmospheric contributions from aerosol asymmetry, matters, e.g. by comparing clear-sky ocean-only contributions. It is correct that these papers are not as frequently cited, but they do contradict the view that there is a community misconception to be corrected. -Datseris and Stevens (2021) is indeed a recent paper that actually focuses on interhemispheric albedo symmetry that does not mention atmospheric contributions, but only land-sea differences as reason for clear-sky asymmetry. Although it may be prominent, to date it also has only one citation. That Datseris and Stevens (2021) doesn't mention aerosol, when based on then available literature it should, is more a criticism of that particular paper. This may be warranted, but should be formulated as such. It would be more fair to say that the original explanation for clear-sky albedo asymmetry is landsea distribution, referring to the early studies in the 70's and 80's, but that after that there has been work pointing at surface as well as atmospheric aerosol contributions, although there are also examples of recent work that fail to acknowledge aerosol influence on clear-sky asymmetry. The current work would build on the existing literature, to confirm those results that include atmospheric contributions, and offer closer quantification of both aerosol and cryosphere contributions to the clear-sky asymmetry, and discussion of the implications of the transient nature of the clear-sky asymmetry, by studying effects of varying aerosol forcing and warming scenarios.
Here are some concrete suggestions for minimum level changes to be made to acknowledge the state of knowledge in the field: Abstract: I would suggest to remove the sentences "The most-cited explanation…" and "However, it is the atmosphere…" from the abstract. This would avoid the implication that the community currently has the wrong view, and that problem is now fixed, and rather keep the focus on the novel findings of the paper.
Line 54 Please rephrase, as discussed above. You may well say the original explanation was the distribution of continents, but later work has pointed at clear-sky asymmetries related to aerosol, explicitly or implicitly.
Line 63 Please change this formulation, as it is (following the above required changes) unclear what the "However" is contrasting with Other minor comments Line 74: that there "would be hard-to-predict ripple effects across the climate system" is both vague and dramatic. Please either rephrase or make concrete.
Line 182: It seems that less polluted and less icy is also an option, they are not necessarily anticorrelated.
Line 253: there could be important implications

(No additional references)
We thank the three reviewers for a second round of thorough and constructive comments. Below, we provide a point-by-point response and explanation of the changes we made to address their concerns. In particular, we would like to highlight that we have fully removed any implications that prior work did not acknowledge atmospheric aerosol as playing a role in the clear-sky asymmetry and have clarified that our contribution is in looking both at the anthropogenic portion of the aerosol problem and at how the cryosphere offsets much of the continent-based surface contribution, and how these anthropogenic aerosol and cryosphere components change over time. Original reviewer comments are in black and author response is in blue.

Reviewer #1 (Remarks to the Author):
Review of "On the rise and fall of Earth's strong clear-sky hemispheric albedo asymmetry" by Michael Diamond et al.
The motivation for the study is the present-day hemispheric symmetry in TOA reflected shortwave radiation (i.e., planetary albedo), which is a fascinating yet not well understood feature of Earth's climate. If the symmetry was the result of some intrinsic compensation mechanisms, this would put a profound constraint on Earth climate and would have important implications for past and future regional changes. Somewhat in contrast to previous work, the authors focus on the clear-sky aspect of the problem, i.e., the quite substantial hemispheric asymmetry in the clear-sky albedo without clouds that makes the all-sky symmetry so fascinating.
The paper is well written and and the analysis methods appear valid. I have already reviewed the previous version of the paper that was under review at Nature. In the revised version considered here the authors present a long rebuttal letter, but the the actual changes to the manuscript in my view are rather small and most of the reviewer's criticism is discussed away (instead of substantial changes to the manuscript). As a result, I remain to feel that the authors are overselling their results and that their work would fit better into a specialized journal, or maybe into a perspective/opinion article. I hope to explain this in the following.
In the previous version I have criticized that the authors have, maybe without wanting to do so, created the idea that previous work has not recognized the leading role of aerosols in setting the clear-sky albedo asymmetry in the present-day climate. In the revised version, the authors now talk of a traditional, most-cited and/or original view that they claim to here find to be insufficient (e.g., L17 and L70 of the tracked-changes manuscript). I find this wording is still trying to create the picture that the authors are here correcting a "wrong" view that in my view simply does not exist. I thus remain unsatisfied with the broader framing of the paper.
We have now eliminated these statements and any implications that the prior literature did not acknowledge aerosol as a factor in the clear-sky asymmetry and are clear that our novel contribution is focusing on the anthropogenic component of the aerosol and on the cryosphere offset to the continental effect in the present day and on past and future changes.
The clear-sky albedo diagnostic itself and the finding of its transient nature are interesting, and the combination of satellite observations and climate model analysis (including historic and future warming runs) makes a good case about the (likely) time dependence of the clear-sky asymmetry, although the analysis method is fairly standard and straightforward. The numbers derived here are subject to quite large uncertainties (i.e., the pre-industrial clear-sky asymmetry is estimated to be within ~0-4 Wm-2, which does not seem a strong constraint), but nevertheless these numbers are helfpul reference points.
The paper closes with a discussion about the possible meaning of the transient nature of the clear-sky asymmetry. This discussion is necessary speculative as it hinges on the unanswered question of whether the all-sky symmetry is pure chance or an intrincic feature, and so gives little further insights. This part might actually fit better into a perspective or opinion piece instead of an original research article.
We believe the two paragraphs in question are helpful for establishing why readers of CEE should care about whether Earth's albedo symmetry is maintained or not. Although our results by their nature (being clear-sky) cannot establish a cloud-related mechanism, they suggest that we will either have a negative answer as to whether such a mechanism exists or will be affected by one within a few decades.

Reviewer #2 (Remarks to the Author):
The authors investigate clear-sky northern hemisphere and southern hemisphere albedo asymmetry using CERES data and CMIP6 outputs. They separate atmosphere and surface contributions using a simple one-layer model. They show that the atmosphere contributes more than surface to the clear-sky hemispherical albedo asymmetry. As a consequence, when aerosol loading in the northern hemisphere decreases, clear-sky albedo asymmetry is reduced. They show that this is the case using two 15-year periods using CMIP6 runs.
The manuscript is well written and understanding results is straightforward. This is in part, as the authors acknowledged in the introduction section, that a larger contribution of the atmosphere to the clear-sky hemispherical albedo asymmetry has been pointed out in earlier studies. therefore, a smaller clear-sky hemispherical albedo asymmetry as the aerosol loading in the northern hemisphere being reduced is a corollary to the earlier result. Therefore, the results are not entirely novel.
I have three minor comments. The third one can be major.
The authors link this clear-sky hemispherical albedo asymmetry to all-sky albedo symmetry and argue on line 250 that, "without any compensation mechanism, we should observe an asymmetry in all-sky reflection in the next few decades". This is true when clouds do not change. However, because cloud signal is much larger compared to aerosol, all-sky asymmetry can be maintained by a separate mechanism regardless of the size of the clear-sky albedo asymmetry. Earth may have a way to maintain symmetrical hemispherical albedo by manipulating clouds because all-sky energy input to each hemisphere may need to be balanced.
We agree with the reviewer and intended to say as such originally. We have rewritten for clarity: "Indeed, without any compensating cloud-related mechanisms, we should observe an asymmetry in all-sky reflection in the next few decades and thus obtain a definitively negative answer to the question of whether Earth's hemispheric all-sky albedo symmetry is maintained. However, if clouds respond to the changing clear-sky contrast to maintain all-sky symmetry over the coming decades, there could be important implications for radiative forcing and hydrological and circulation changes depending on the (currently unknown) adjustment mechanism." Because the way of authors compute surface and atmosphere contributions, both contributions change when aerosol loading changes. Two contributions are not entirely independent, although the change in the surface contribution is at least one order of magnitude smaller than the change in atmospheric contribution. This can be seen by taking a derivative of Eq. (1) with respect to alpha_atm. The authors need to recognize this point.
Thank you for pointing this out. We have now added a discussion within the Methods: "As is clear from Equations 1 and 2, the surface and atmospheric components are not independent because the surface component depends on the atmospheric albedo and transmissivity (which accounts for both scattering and absorption). Surface component changes resulting from changes in the atmosphere are much smaller than the original atmospheric changes, however. Thus, while changes in atmospheric aerosol do affect both the atmospheric and surface components, they primarily affect the atmosphere." If only the transmission changes, such as by decreasing soot loading without changing alpha_atm (i.e. changing absorption coefficient without changing scattering coefficient), then only surface contribution changes. Because of the way authors compute contributions, the change of the contribution by aerosol change depends on what type of aerosol changes. A smaller clear-sky hemispherical albedo symmetry during the 1850-1865 period depends on the type of aerosols used in CMIP6 runs. I doubt that the sensitivity to aerosol type is large enough to changes conclusion. But at lease the authors need to discuss that the contribution is sensitive to aerosol type.
Thank you for pointing this out. We have now added discussions of this point twice in the results section and a new Extended Data Figure showing the aerosol change by type for the seven AerChemMIP models.
CERES/MERRA-2 section: "It should be noted that absorbing aerosol like dust and black carbon increase atmospheric reflection less than would fully scattering particles, and because they efficiently reduce transmissivity, will further reduce surface reflection as well. Our results are therefore partially sensitive to aerosol type, although this effect is not expected to be large enough to materially affect any conclusions, at least on the hemispheric scale. The effect may be important for regions with particularly large black carbon or dust changes, however." Pre-industrial section: "Changes in Dτa are driven primarily by increasing sulfate aerosol in the NH (Extended Data Fig. 5)." Reviewer #3 (Remarks to the Author): Review of On the rise and fall of Earth's strong clear-sky hemispheric albedo asymmetry, by Diamond et al. This is a revised version of the manuscript, that scrutinizes the contributions to interhemispheric asymmetry in clear-sky albedo in satellite observations, and in future projections. It highlights that the current asymmetry is transient, due to the role of cryosphere and atmospheric aerosol in producing it, and discusses implications of potential cloud adjustment to the clear-sky asymmetry.
The topic of interhemispheric albedo symmetry is fascinating, and closer investigation of the underlying clear-sky asymmetry is useful. The findings are interesting, and clearly presented. The investigation of the clear-sky albedo symmetry response under different future scenarios, with varying warming and aerosol loadings is novel and relevant, although the discussion of possible cloud adjustments remains speculative, since they rest on the question whether clouds will adjust or not. My specific comments regarding the regression in Fig 3, the discussion of the COVID lockdown analogy, and Methods description have been addressed in a satisfying way.
We thank the reviewer again for their useful suggestions with all these points and are pleased that our responses have satisfied their concerns.
Unfortunately, however, the authors have failed to address the central concern in my previous review: the poorly founded claim that this study corrects a prevailing misconception in the community, that the clear-sky albedo asymmetry is determined by land-ocean distribution. The manuscript actually changed very little in this respect, and reading the new version, I see the same message, that this work claims to be the first to point at other factors than land-ocean distribution determining the clear-sky albedo asymmetry. As before, this is not correct, and the authors need to re-frame theirindeed interesting and relevant -results to align with the published literature on the topic. In the current framing, I don't think their work should be published.
We did not intend to leave this impression in the revised work and agree with the reviewer that we are definitely not the first to point this out in general. We agree that our phrasing here has become an unnecessary distraction from our main point (the transient nature of the asymmetry) and therefore have decided to avoid discussing anything that could be construed as a "misconception in the community." We have revised the text in line with the useful suggestions provided below.
Formulations in the paper have changed from "The traditional explanation…" to "The most cited explanation…" (in the abstract) and "The traditional and most-frequently invoked explanation…" (in the introduction), with references to Vonderhaar and Suomi (1971), Ramanthan (1987), Stephens et al (2015, 2016 and Datseris and Stevens (2021), but this does not solve the problem.
Neither the paper, nor this review of it is a meta-study of the literature and citation paths on this topic, but a brief search shows that this account is at best incomplete.
- Vonderhaar & Suomi (1971) and Ramanathan (1987) indeed point at surface features being compensated by clouds, and are indeed very well cited (more than 200 and more than 400 citations respectively, although likely not only for pointing at the interhemispheric albedo symmetry).
-Stephens et al (2016) (36 citations) also refers to "…the surface radiative fluxes that are slightly more asymmetrical", but without reference to the mentioned Ramanathan (1987) or Vonderhaar&Suomi (1973) in that context. -Stephens et al (2015), as the authors agree, discuss and quantify contributions to clear-sky asymmetry arising from differences in aerosol distribution (their section 4). This paper, that arguably is central for bringing the interhemispheric symmetry into discussion in more recent times, has 191 citations.
-In addition, as the authors also agree but still leave out in introducing their topic and motivating their study, both earlier Voigt et al (2013) (39 citations), and later Bender et al (2017) (11 citations), and Jonsson and Bender (2021) (1 citation) show or clearly state (and stating should be enough to count for a general argument of the type "most cited view") that atmospheric contributions from aerosol asymmetry, matters, e.g. by comparing clear-sky ocean-only contributions. It is correct that these papers are not as frequently cited, but they do contradict the view that there is a community misconception to be corrected. -Datseris and Stevens (2021) is indeed a recent paper that actually focuses on interhemispheric albedo symmetry that does not mention atmospheric contributions, but only land-sea differences as reason for clear-sky asymmetry. Although it may be prominent, to date it also has only one citation. That Datseris and Stevens (2021) doesn't mention aerosol, when based on then available literature it should, is more a criticism of that particular paper. This may be warranted, but should be formulated as such. It would be more fair to say that the original explanation for clear-sky albedo asymmetry is land-sea distribution, referring to the early studies in the 70's and 80's, but that after that there has been work pointing at surface as well as atmospheric aerosol contributions, although there are also examples of recent work that fail to acknowledge aerosol influence on clear-sky asymmetry. The current work would build on the existing literature, to confirm those results that include atmospheric contributions, and offer closer quantification of both aerosol and cryosphere contributions to the clear-sky asymmetry, and discussion of the implications of the transient nature of the clear-sky asymmetry, by studying effects of varying aerosol forcing and warming scenarios.
Here are some concrete suggestions for minimum level changes to be made to acknowledge the state of knowledge in the field: